A lithium-ion secondary battery shows the highest energy density in commercially available batteries, and is often used particularly for small electronics. Also, in addition to the small intended purposes, it is expected to apply to an automobile. Among these, a lithium-ion secondary battery is required to increase capacity, extend lifetime and further improve safety.
A polyolefin, such as polyethylene and polypropylene, based organic separator is generally used in the lithium-ion secondary battery for preventing short circuit between a positive electrode and a negative electrode. Since the polyolefin based organic separator is melted at 200° C. or lower, volume change such as contraction and meltdown can be caused when the battery is heated to a high temperature due to inside and/or outside stimuli, resulting in short circuit between the positive electrode and the negative electrode, release of electrical energy, and the like which may cause explosion, etc.
To solve these problems, it has been proposed to form a layer including a nonconductive particle such as inorganic particle on the polyethylene organic separator or electrode.
For example, Patent Document 1 discloses a use of a porous membrane, comprising nonconductive particle and resin binder and adhered to at least either surface of positive electrode or negative electrode, as a separator. The porous membrane is formed by coating slurry comprising nonconductive particles and resin binder dissolved in a solvent, and then drying thereof. As for the resin binder used for the slurry, fluorine resin, polyolefin resin, etc. are exemplified.
Also, Patent Document 2 discloses a protective porous membrane formed by using microparticle slurry including nonconductive particle such as alumina, silica, polyethylene resin on an electrode.
Furthermore, Patent Document 3 discloses a method preventing internal short circuit due to dropping of an electrode material mixture generated during the manufacturing process of a battery with a porous membrane, by controlling expansion rate of the porous membrane or controlling distribution state of resin binders in the porous membrane in a thickness direction.
However, as is the same with Patent Document 1, even with Patent Documents 2 or 3, strength of a porous membrane is insufficient and that it may be easy to break and the porous membrane may drop from a polar plate during the manufacturing process of a battery when coating nonconductive particles such as nonorganic microparticles on an electrode without dropping. Thus, safety deteriorates which leads to induce of an internal short circuit and production yield of a battery decreases. Particularly, in case of a rolled-up shaped lithium-ion secondary battery, positive electrode and negative electrode are spirally rolled-up intervening a separator between both electrodes. A curvature radius is small at a start part of the rolling and that bending stress become large and a porous membrane is likely to break.
Accordingly, with Patent Documents 1 to 3, an electrical short circuit can be prevented and thermal contraction can be controlled by forming porous membrane including nonconductive particle such as inorganic particles. However, the strength of a porous membrane is insufficient and that it may be easy to break and the porous membrane may drop from a polar plate during the manufacturing process of a battery, thus, safety deteriorates which leads to induce of an internal short circuit and production yield of a battery decreases. Further, with any Patent Documents, it is difficult to further increase the strength of a porous membrane maintaining lithium-ion conductivity.    [Patent Document 1] Japanese Unexamined Patent Publication No. H. 10-106530 (corresponding U.S. Pat. No. 5,948,464)    [Patent Document 2] Japanese Unexamined Patent Publication No. H. 7-220759    [Patent Document 3] WO Publication No. 2005-011043 (corresponding U.S. Pat. No. 7,396,612)